Control apparatus for steam heating systems



Dec. 8, 1953 CONTROL V. D. WISSMILLER APPARATUS FOR STEAM HEATING SYSTEMS Filed March 30, 1951 AMPLIFIER INVENTOR. VERNON D WISSMILLER BY 14 M ATTORNEY Patented Dec. 8, 1953 CONTROL APPARATUS FOR STEAM HEATING SYSTEMS Vernon Wissmiller, Minneapolis, Minn, as-

signor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn, a corporatlonof Delaware Application March so, 1951, Serial No. 218,294

' F (01. zen-9') 7 I 7 3 Claims.

, 1 s The present invention relates to temperature control apparatus for a steam heating system.

Conventionally, steam heating systems are controlled, at least in part, by pressure responsive devices with a thermostat of some sort exercisin dominant control. These prior art arrangements are generally efiective but costly. Further, in cases where the steam supply is superheated, a pressure measurement of the steam does not ac- .curately reflect its heating ability. some attempts have been made to control steam heating systemsentirely by temperature responsive devices but, where it is necessary to determine a.

return line temperature, the results have not been satisfactory because of a tendency for the return line 'to become sub-cooled due to its exposure conditions. 7 g

The present invention overcomes these difliculties by averaging the supply temperature and by providing a temperature responsive device exposed tosaturated steam at the pressure of the return line. By this arrangement,-temperature of the supply is accurately taken into account and a temperature is obtained for the return line that properly indicates the influence of return pressure on the system but which is unaffected by the exposure conditions of the return line.

It is thus a major object of this invention to pro-- vide improved control apparatus for a steam heating system. r

It is a further and more specific object to provide'a practical differential temperature control system for a steam heating system wherein the differential may be maintained constant ormay be varied as desired. s

It is an additiona1 object to provide means for establishing a temperature which accurately refiects the return line pressure. 7

These and other objects will become apparent upon a study of the following specification and drawings in which: r

Figure 1 is a schematic showing of a steam heating system equipped with the present control apparatus.

Figure 2 is a modified network circuit which may be used in the system of Figure 1.

In the system shown in Figure 1. steam is supplied through main ill from a suitable boiler or central heating system, the flow of steam being controlled by a motor valve H. The steam is taken from the main II] to the various radiators, l2, I3 and [4 through individual risers and orifices in a conventional manner, these radiators being representative of a conventional steam heating system. In addition radiators l2, l3 and '2 I4 are connected through suitable traps and return risers to a. return main it which extends down to the basement floor l8 and runs along this floor until it reaches condensate pump I! I from which it is delivered to the boiler. Pump I9 is preferably controlled by its own controller, not shown, responding to return linepressure. While the present system is shown with orifices and traps, it seems obvious that other means for controlling the steam distribution among the various radiators may be used and the traps may be eliminated if desired, neither of these features being necessary in the performance of this invention. The motor 2| which operates motorvalve l I is preferably of the proportioning type and includes a follow-up potentiometer 22 having a wiper 23 movable over a resistor 24.

Motor 2| is controlled by amplifier 26 which may be of any suitable type capable of discriminating the phase relations of its input signals so that it can properly control a reversible motor such as 2|. As an example of a motor and amplifier found suitable in this apparatus, reference is made to Patent N 2,423,5s4 issued to Albert P. Upton July 8, 1947. As in the Upton patent, amplifier 26 is controlled by a bridge type network circuit generally identified by the numeral 21, network 2-! having input terminals 28 and 29' connected to the secondary winding of a. transformer 30, and the'network also includes an output terminal 3| with wiper 23 constituting the other output terminal. Output terminal 3| is connected to the input of amplifier 26 by wire 32 and wiperv 23 is connected to the input of the amplifier by the wire 33. The upper left hand branch of network 21 comprises a manually adjustable resistor 35 and a temperature responsive proximati'ng in resistance value the fixed resistorv 31. Chamber 4| is essentially only a sufficiently enlarged portion of conduit 48 to receive well '40 andto permit steam to surround the well. The

lower; right hand branch of thenetwork com? prises a -temperatureresponsive resistor" receivedin aweli 43 inserted in main I 0 and also includes a similar fixed resistor 44 in well 45 which is inserted near a remote portion of main III. For the purpose of this illustration, the total resistance value of resistors 42 and 44 approximates that of resistors 35 and 36, under normal conditions. Resistor 24 is connected across the lower branches of the networkand an adjustable resistor 41 is connected in shunt relation to resistor 24.

The steam chamber 4| in which resistor 39 is located is connected to supply main ID by a conduit 48, an orifice or similar restriction 49 limiting the flow of steam through this conduit, this orifice being only large enough to avoid plugging for only a small fiow of steam is needed for element 39. In addition, suitable fins or the like 50 may be attached to conduit 48 to insure that some condensation will take place in the conduit before reaching chamber 4!, to thereby assure that saturated steam will be provided for chamber 4|. As an alternative to fins 50, the conduit between orifice 49 and chamber 4| may be lengthened or other provisions made to insure some condensation in the conduit. The bottom portion of chamber 4| is connected through conduit 52 to return main |6 upstream from the suction side of pump I9. Fins 53 or the like are used on conduit 52 to make certain that all of the steam remaining in the conduit after passing through chamber 4| will be condensed before reaching pump l9. While fins 5|! and 53 are used on conduits 48 and 52, respectively, it is obvious that any means of insuring sufficient condensation will be adequate and it is unimportant if some steam still remains in conduit 52 if the connection to the return line I6 is far enough away from pump |9 so that the condensate flowing in main IE will condense the steam before it reaches the pump so that operation of the pump will not be impaired.

With the arrangement above described, it will be apparent upon inspection that network 21 is arranged to maintain a predetermined balance in resistance between the lower portions of the bridge, assuming that the upper portions are fixed. However, upon a change in resistance of either resistors 35 or 35 or both, then a difierent state of balance must be maintained in the lower branches of the network to keep the bridge in balance so that the system, as shown, operates to vary the operating differential temperature of the system upon a change in room temperature.

Thus, a lower room temperature will call for anincreased temperature differential and a higher room temperature will call for a decreased temperature diiferential in the system. If desired, resistor 35 may be made part of an outdoor'controller or other such device for determining system load, a room temperature device being shown for the purposes of illustration.

A slightly modified arrangement of the above system is sometimes desirable, especially for zone control, and network circuit 55 shown in Figure 2 shows schematically a modification. In this case, assuming that the input and output terminals are the same as above, the upper left hand branch may comprise manually adjustable resistor 35 and temperature responsive resistor 39; the upper righthand branch includes temperature responsiveresistors 42 and 44 and the lower branches of the network include fixed resistors 58 and 59, with resistor 24 of follow-up potentiometer 22 being connected acrossthese lower branches as inthe preceding example. With this arrangement, the system will control to maintain a fixed differential of temperature between the supply resistors and the return temperature responsive resistor,

- urated steam in the chamber 4|.

'4 with the valve of the differential being determined by the manually adjustable resistor 35.

Operation To more fully illustrate the present invention, its operation will now be discussed. Assuming that steam is flowing through main Ill, valve H is partly open and radiators l2, l3 and I4 are receiving steam and discharging condensate through return main IE, it may further be assumed that the system is in equilibrium. With pump is running and, as usual in such systems, under the control of a return line pressure controller, it operates to maintain the return line pressure between predetermined values. These predetermined values may comprise a rather narrow range of pressure or, if less frequent starting and stopping of the pump I9 is desired, the values may be somewhat more'widely separated. With the system in operation as described, steam is not onlyfiowing through the conventional portions of the system in the ordinary fashion, but it is also flowing through orifice 49 into conduit 48 in rather small quantities, some of this steam being condensed due to the cooling effect of fins 50, with the steam and condensate being delivered to chamber 4| which surrounds well 48, so that resistor 39 responds to the temperature of the sat- However, because conduit 52 is connected to the bottom of chamber 4| and is also connected to main l6 and has no appreciable restrictions in its length, the

pressure in chamber 4| is substantially the same as that in return line l6, hence bulb 39 is responding to a temperature of saturated steam existing at the pressure in the return line and, as is well known, where saturated steam is used, there is a definite relation between pressure and temperature under all conditions. Assuming that pump 9 is running, it is obvious that it will tend to reduce the pressure in line I6 and, when this a different voltage and there is a predetermined phase relation between them. This is effective to cause amplifier 26 to control motor 2! in a direction to reduce steam fiow to thereby reduce the temperature at resistors 42 and 44, thus tending to bring the bridge back in balance. In addition, as the motor 2! operates, wiper 23 is adjusted over resistor 24 in a direction to reduce the unbalance of the network, as is common in proportioning circuits. Depending on the speed of operation or motor valve and the response of the system to adjustments of the valve, resistor 4! is adjusted to give only enough authority to potentiometer 22 to keep the system stable and, in many cases, resistor 41 may be adjusted to a position in which potentiometer 22 has no effect so that the system then becomes, in effect, a floating type of control apparatus. Regardless of whether the system operates in a floating manner or in a proportioning manner, it is obvious that as the return tem perature goes down, the supply temperature must also go down, assuming that the room temperature responsive resistor and the fixed resistor 35 remain unchanged. As these temperature differ .ences reflect pressure differences, it is obvious th t he te of flow of steam into the radiators then remains constant because the rate of flow is essentially a function of pressure difference and the tctal output of the heating system is much more sensitive to changes in the rate of flow than it is to minor variations in the temperature of the steam being supplied.

Upon the pump 59 stopping, it is obvious that the pressure in the return line IE will tend to build up gradually so that the pressure in 4| likewise builds up and so does the temperature, with a resultant increase in resistance of resistor 39. The increase in resistance of 39 tends to unbalance the bridge in an opposite direction from that previously experienced with the result that amplifier 25 controls motor 2| in a direction to increase the flow of steam and thereby raise the temperature of resistors 42 and 44 and, as a result, by keeping the differential temperature constant the diiferential pressure is also kept constant with the rate of flow of steam through the radiators being maintained in equilibrium.

If the temperature of resistor 36 should now decrease, the network will be unbalanced in a direction requiring an increase in resistance of the lower right hand branch or a decrease in the resistance of the lower left hand branch, or both, to restore the balance and, until the balance is restored, the network feeds signals to amplifier 25 of a sort causing motor 2| to adjust valve ll in a direction to increase the flow of steam, this increase causing a rise in temperature of resistors 42 and 44 to thereby restore the balance of the network. In a similar fashion, an increase in temperature of resistor 36 will cause an unbalance of the network in a direction requiring less resistance in the lower right hand branch of the network or an increase in the temperature of the lower left hand branch, or both, to thus lower the differential temperature being maintained; lower the differential of pressure across the radiators, and thereby cause a lesser rate of heat generation in the spaces.

In the above discussion, it seems clear that resistors 42 and 44 act in an averaging manner but, where the system supply main is generously sized so that uniform pressures are maintained throughout the main, these two resistors may be combined in a single device of equal authority without afiecting the operation of the system at all providing that the single resistor and its well be placed far enough down the stream from the valve to not be unduly affected by the normal super heat resulting when steam is expanded or, in cases where considerable superheat is used, a

single resistor may be used if placed at a representative spot. However, where there may sometimes be considerable superheat in the steam at the discharge side of the valve, the averaging bulbs are preferred with the downstream bulb preferably having more authority in the system, due to more resistance, than the upstream bulb.

In operation, the system function using the 6 network 56 shown in Figure 2 will be the same as that above discussed with the exception that the differential maintained is normally adjusted only by resistor 35, the system operating to maintain a fixed differential of temperature at all times.

As a study of the preceding specification and drawings will suggest many substitutions and equivalents, the scope of this invention should be determined only by the following claims.

I claim:

1. In a steam heating system including heat exchange means for a space and having a source of steam and a return means, means for controlling the supply of steam from said source, pump means for insuring the return of condensate through said return means and for determining the return means pressure, said return means being of such nature that under at least some'conditions of operation the condensate in the return means is sub-cooled, a chamber connected to said return means in such manner that the pressure therein corresponds to that in said return means, a restricted conduit connected between said source of steam and said chamber, said conduit including sufficient heat exchange surface to condense a portion of the steam supplied to said chamber from said source, means responsive to the temperature of the steam supplied from said source, means responsive to the temperature in said chamber, and means connecting both of said temperature responsive means in joint controlling relation to said means controlling the supply of steam, said connecting means being arranged to control said supply controlling means in a manner to maintain a predetermined difference in temperature between the temperature responsive means of said supply and of said chamber, said connecting means including means for varying said predetermined temperature difference.

2. A system as recited in claim 1 wherein the means for varying said predetermined temperature diiference comprises a device responsive to the temperature of said space.

3. A system as recited in claim 1 wherein the means for varying said predetermined temperature difference comprises manually operable means.

VERNON D. WISSMILLER.

References Cited m the file of this patent UNITED STATES PATENTS Number Name Date 550,814 Barrus Dec. 3, 1895 1,100,077 Hornung June 16, 1914 1,867,849 Kuenhold July 19, 1932 2,090,073 Rohlin Aug. 17, 1937 2,236,624 Littwin Apr. 1, 1941 2,549,621 Moore Apr. 17, 1951 

